Print line above pattern in python

Input file: 'data.txt' contains results of a set of students in the following format.

$ cat data.txt
id:502
Result:Pass
id:909
Result:Fail
id:503
Result:Pass
id:501
Result:Pass
id:802
Result:Fail

Required:
Print the Ids of the students who have passed the exam.

The python program:

fp = open("passedids.txt","w")
data = open("data.txt").readlines()
for i in range(len(data)):
        if data[i].startswith("Result:Pass"):
                fp.write(data[i-1].split(":")[1])

Output:

$ cat passedids.txt
502
503
501